Method and Structures for Acoustic Wave Overlay Error Determination

ABSTRACT

A method includes forming a first material layer on a semiconductor wafer, the first material layer comprising a first periodic structure within an overlay mark region of the semiconductor wafer and forming a second material layer on the semiconductor wafer, the second material layer comprising a second periodic structure in the overlay mark region. The method further includes with an acoustic transmitter device disposed within the overlay mark region, transmitting an acoustic wave across both the first periodic structure and the second periodic structure. The method further includes, with an acoustic wave receiver device, detecting the acoustic wave and determining an overlay error between the first material layer and the second material layer based on the acoustic wave as detected by the acoustic wave receiver device.

PRIORITY INFORMATION

This application is a divisional of U.S. patent application Ser. No.15/962,518 filed Apr. 25, 2018 and entitled “Method and Structures forAcoustic Wave Overlay Error Determination,” which claims priority toProvisional Patent Application No. 62/592,241 filed Nov. 29, 2017, andentitled “Methods and Structures for Acoustic Wave Overlay ErrorDetermination,” the disclosure of which is hereby incorporated byreference in the entirety.

BACKGROUND

In the semiconductor integrated circuit (IC) industry, technologicaladvances in IC materials and design have produced generations of ICswhere each generation has smaller and more complex circuits than theprevious generation. In the course of IC evolution, functional density(i.e., the number of interconnected devices per chip area) has generallyincreased while geometry size (i.e., the smallest component (or line)that can be created using a fabrication process) has decreased. Thisscaling down process generally provides benefits by increasingproduction efficiency and lowering associated costs. Such scaling downhas also increased the complexity of IC processing and manufacturing.

One challenge to semiconductor fabrication is alignment. Semiconductorfabrication involves forming several patterned layers on top of eachother. Each of these layers must be precisely aligned, or else the finaldevice may not function correctly.

Alignment techniques often involve the use of overlay marks. Forexample, various layers to be patterned on a substrate may includeoverlay marks that are used to align with other formed layers. Matchingoverlay marks are formed within patterns of the subsequently formedlayers. These matching overlay marks are placed within the patterns ofthe subsequent layers such that when aligned with the correspondingoverlay marks of the underlying layers, both layers are aligned. But,such alignment techniques are not perfect and it is desirable to havealignment techniques that provide improved alignment.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a diagram showing an illustrative overlay mark structure,according to one example of principles described herein.

FIGS. 2A, 2B, and 2C are diagrams showing a top view of periodicstructures used for overlay marks, according to one example ofprinciples described herein.

FIGS. 3A, 3B, and 3C are graphs showing acoustic wave signalscorresponding to the placement of the periodic structures of FIGS. 2A,2B, and 2C, according to one example of principles described herein.

FIGS. 4A, 4B, and 4C are graphs showing reflected acoustic wave signalscorresponding to the placement of the periodic structures of FIGS. 2A,2B, and 2C, according to one example of principles described herein.

FIGS. 5A, 5B, and 5C are graphs showing transmitted acoustic wavesignals corresponding to the placement of the periodic structures ofFIGS. 2A, 2B, and 2C, according to one example of principles describedherein.

FIGS. 6A and 6B are diagrams showing illustrative feature shapes foroverlay marks, according to one example of principles described herein.

FIGS. 7A, 7B, 7C, 7D, 7E, 7F, and 7G are diagrams showing variousconfigurations for overlay marks, according to one example of principlesdescribed herein.

FIG. 8 is a diagram showing various periodic structures, acoustic wavetransmitters and acoustic wave receivers, according to one example ofprinciples described herein.

FIG. 9 is a diagram showing multiple overlay mark regions in series,according to one example of principles described herein.

FIG. 10 is a diagram showing multiple overlap mark regions in parallel,according to one example of principles described herein.

FIGS. 11A to 11D are diagrams showing illustrative formation of overlaymarks, according to one example of principles described herein.

FIG. 12 is a flowchart showing an illustrative method for using acousticwaves to detect overlay error, according to one example of principlesdescribed herein.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

As described above, alignment techniques often involve the use ofoverlay marks, which are sometimes referred to as overlay marks. Forexample, various layers to be patterned on a substrate may includeoverlay marks that are used to align with other formed layers. Matchingoverlay marks are formed within patterns of the subsequently formedlayers. These matching overlay marks are placed within the patterns ofthe subsequent layers such that when aligned with the correspondingoverlay marks of the underlying layers, both layers are aligned. But,such alignment techniques are not perfect and it is desirable to havealignment techniques that provide improved alignment.

According to principles described herein, overlay marks from twodifferent layers are designed to transmit or reflect acoustic waves.Such overlay marks may be placed in a region of a semiconductor where atransducer is placed. The transducer converts electrical energy tomechanical energy in the form of an acoustic wave. The acoustic wavethen passes through a first overlay mark associated with a first layerand a second overlay mark associated with a different layer. Theacoustic wave, as either reflected off the overlay marks or transmittedthrough the overlay marks, may then be detected by an acoustic wavereceiver device. The nature of the detected wave will change based onthe alignment between the first overlay mark in the second overlay mark.Thus, by analyzing the detected acoustic wave, and alignment error maybe determined.

FIG. 1 is a diagram showing an illustrative overlay mark structure 100.The overlay mark structure 100 is positioned within a particular region102 within a semiconductor chip. The overlay mark structure 100 may bepositioned within one or more layers of a semiconductor wafer. Forexample, the overlay mark structure 100 may be positioned on thesemiconductor substrate or any subsequently formed layer such as a metallayer or dielectric layer. According to the present example, the overlaymark structure 100 includes an acoustic wave transmitter device 104, andacoustic wave receiver device 106, the periodic structure region 112having a first periodic structure 108 and a second periodic structure110.

The semiconductor wafer in which the overlay mark is positioned may be acircular wafer used to fabricate integrated circuits. In some examples,the overlay mark structure 100 may be positioned within the scribe linesof the semiconductor wafer. The scribe lines are the lines on which thesemiconductor wafer is cut after fabrication. In some examples, however,the overlay mark structure 100 may be positioned between scribe linesand thus be part of the final semiconductor chip product.

The acoustic wave transmitter device 104 may be an accusing emitter. Inother words, the acoustic wave transmitter device 104 is designed toemit an acoustic wave along the surface of the substrate. In oneexample, the acoustic wave transmitter device 104 is an interdigitaltransducer. The acoustic wave transmitter device 104 may be designed toconvert electrical signals into mechanical signals. In some examples,the overlay mark structure 100 may be positioned over a piezoelectriclayer. A piezoelectric material may experience mechanical strain orstress upon application of an electrical current. Thus, an AC electricalsignal applied to the acoustic wave transmitter device 104 may causesurface acoustic wave signal to be transmitted across the periodicstructure region 112.

The periodic structure region 112 includes the first periodic structure108 and the second periodic structure 110. In one example, the firstperiodic structure 108 is formed along with a first patterned layerformed on the semiconductor wafer. For example, the first patternedlayer may be a polysilicon gate layer. Thus, the first periodicstructure 108 may thus include polysilicon features. Such features maybe formed through photolithographic processes. For example, apolysilicon layer may be deposited. A photoresist material may then bedeposited over the polysilicon layer. The photoresist material may thenbe exposed to a light source through a photomask and then developed. Theexposed regions of the polysilicon layer may then be removed through anetching process to create the patterned polysilicon layer. As will beexplained in further detail below, the first periodic structure 108 mayinclude a two-dimensional array of features. Such features may be sizedand spaced apart so as to create a desired frequency profile for anacoustic wave being transmitted through or reflected off of the periodicstructure 108.

The second periodic structure 110 may be similar to the first periodicstructure 108. Specifically, the second periodic structure 110 mayinclude a two-dimensional real features being sized and shaped similarlyto those of the first periodic structure 110. The second periodicstructure may be associated with a second material layer formed on asemiconductor wafer. The second periodic structure 110 is positionedwithin the overall pattern such that when the pattern of the secondmaterial layer is aligned with the pattern of the first material, thesecond periodic structure 110 is positioned adjacent the first periodicstructure 108. The second periodic structure 110 is positioned adjacentthe first periodic structure 108 such that when properly aligned, thefeatures from both structures 108, 110 form a single two-dimensionalarray with similar spacing throughout.

In the present example, the acoustic wave receiver device 106 ispositioned opposite the periodic structure region 112 from the acousticwave transmitter device 104. Thus, the acoustic wave receiver device 106may be designed to detect acoustic waves as they are transmitted throughthe first periodic structure 108 and the second periodic structure 110.The acoustic wave receiver device 106 is designed to convert mechanicalenergy into electrical energy. In other words, the acoustic wavereceiver device 106 detects the surface acoustic waves being transmittedthrough the periodic structures 108, 110, and converts those surfaceacoustic waves into electrical signal that is representative of theservice acoustic waves. Electric signal can be analyzed forcharacteristics indicative of water the first periodic structure 108 isaligned with the second periodic structure 110.

In some examples, the acoustic wave receiver device 106 may bepositioned on the same side of the periodic structure region 112 as theacoustic wave transmitter device 104. In such examples, the acousticwave receiver device 106 may be configured to detect surface acousticwaves as they are reflected off of the periodic structures 108, 110.

FIGS. 2A, 2B, and 2C are diagrams showing a top view of periodicstructures used for overlay marks. According to the present example,FIG. 2A illustrates two overlay marks 202, 204 that are properlyaligned. The first overlay mark 202 is a periodic structure such as theperiodic structure 108 described above. The second overlay mark 204 isalso a periodic structure such as the periodic structure 110 describedabove.

FIG. 2B illustrates an example in which the overlay marks 202, 204 aremisaligned. Specifically, the second overlay mark 204 is spaced furtherthan it should be from the second overlay mark 204. FIG. 2C illustratesan example in which the overlay marks 202, 204 are misaligned.Specifically, the second overlay mark 204 is spaced closer than itshould be to the first overlay mark 202.

FIGS. 3A, 3B, and 3C are graphs showing acoustic wave signalscorresponding to the placement of the periodic structures of FIGS. 2A,2B, and 2C. Specifically, FIG. 3A illustrates an acoustic signalcorresponding to the positions of the overlay marks 202, 204 as shown inFIG. 2A. FIG. 3B illustrates an acoustic signal corresponding to thepositions of the overlay marks 202, 204 as shown in FIG. 2B. FIG. 3Cillustrates an acoustic signal corresponding to the positions of theoverlay marks 202, 204 as shown in FIG. 2C.

FIG. 3A illustrates a graph with a vertical axis that representsamplitude and a horizontal axis 304 that represents frequency. Thesignal 308 corresponds to an acoustic signal produced when the overlaymarks are spaced as shown in FIG. 2A. The center frequency 306 of such asignal is indicated by the dotted line. FIG. 3B illustrates a signal 310associated with the overlay marks 202, 204 as they are shown in FIG. 2B.In other words, because the overlay marks are spaced apart further thanthey should be, the center frequency of signal 310 is offset from (lessthan) the center frequency 306 of where it should be. Similarly, FIG. 3Cillustrates a signal 312 associated with the overlay marks 202, 204 asthey are shown in FIG. 2C. In other words, because the overlay marks arespaced closer than they should be, the center frequency of signal 312 isoffset from (greater than) the center frequency 306 of where it shouldbe.

FIGS. 4A, 4B, and 4C are graphs showing reflected acoustic wave signalscorresponding to the placement of the periodic structures of FIGS. 2A,2B, and 2C, according to one example of principles described herein.Specifically, FIG. 4A illustrates a reflected acoustic signalcorresponding to the positions of the overlay marks 202, 204 as shown inFIG. 2A. FIG. 4B illustrates a reflected acoustic signal correspondingto the positions of the overlay marks 202, 204 as shown in FIG. 2B. FIG.4C illustrates an acoustic signal corresponding to the positions of theoverlay marks 202, 204 as shown in FIG. 2C.

FIG. 4A illustrates a graph with a vertical axis 302 that representsamplitude and a horizontal axis 304 that represents frequency. Thesignal 402 corresponds to an acoustic signal reflected from the overlaymarks 202, 204 when the overlay marks 202, 204 are spaced as shown inFIG. 2A. The center frequency 306 of such a signal is indicated by thedotted line. FIG. 4B illustrates a signal 404 as reflected off of theoverlay marks 202, 204 as they are shown in FIG. 2B. In other words,because the overlay marks 202, 204 are spaced apart further than theyshould be, signal 404 is offset from (less than) the center frequency306 of the aligned signal 402. Similarly, FIG. 4C illustrates a signal406 as reflected off of the overlay marks 202, 204 as they are shown inFIG. 2C. In other words, because the overlay marks 202, 204 are spacedcloser than they should be, the signal 406 is offset from (greater than)the aligned signal 402.

FIGS. 5A, 5B, and 5C are graphs showing transmitted acoustic wavesignals corresponding to the placement of the periodic structures ofFIGS. 2A, 2B, and 2C. Specifically, FIG. 5A illustrates a reflectedacoustic signal corresponding to the positions of the overlay marks 202,204 as shown in FIG. 2A. FIG. 5B illustrates a reflected acoustic signalcorresponding to the positions of the overlay marks 202, 204 as shown inFIG. 2B. FIG. 5C illustrates an acoustic signal corresponding to thepositions of the overlay marks 202, 204 as shown in FIG. 2C.

FIG. 5A illustrates a graph with a vertical axis 302 that representsamplitude and a horizontal axis 304 that represents frequency. Thesignal 502 corresponds to an acoustic signal after being transmittedthrough the overlay marks 202, 204 when the overlay marks 202, 204 arespaced as shown in FIG. 2A. The center frequency 306 of such a signal isindicated by the dotted line. FIG. 4B illustrates a signal 504 astransmitted through the overlay marks 202, 204 as they are shown in FIG.2B. In other words, because the overlay marks 202, 204 are spaced apartfurther than they should be, signal 504 is offset from (less than) thecenter frequency 306 of the aligned signal 402. Similarly, FIG. 5Cillustrates a signal 506 as reflected off of the overlay marks 202, 204as they are shown in FIG. 2C. In other words, because the overlay marks202, 204 are spaced closer than they should be, the signal 506 is offsetfrom (greater than) the aligned signal 402.

FIGS. 6A and 6B are diagrams showing illustrative feature shapes foroverlay marks. According to the present example, FIG. 6A illustrates afirst overlay mark 602 having a periodic structure adjacent a secondoverlay mark 604 having a periodic structure. Both overlay marks 602,604 are substantially rectangular in shape. More specifically, asillustrated, the overlay marks 602, 604 have a square shape. FIG. 6Billustrates a first overlay mark 612 having a periodic structureadjacent a second overlay mark 614 having a periodic structure. Bothoverlay marks 612, 614 are substantially elliptical in shape. Othershapes are contemplated as well. For example, the features of theoverlay marks described herein may have a circular shape.

FIGS. 7A, 7B, 7C, 7D, 7E, 7F, and 7G are diagrams showing variousconfigurations for overlay marks. According to the present example, FIG.7A illustrates a first overlay mark with a set of features 702 thatcircumscribes, or surrounds features 704 of a second overlay mark. Whileonly a single row of features 702 is shown surrounding the features 704,it is understood that some examples may include several rows of features702 surrounding features 704. When the features 702 are misaligned fromfeatures 704, the characteristics of acoustic waves either beingtransmitted through or reflected off of the features 702, 704 willindicate the nature of the misalignment.

FIG. 7B is a diagram showing an example in which a first overlay mark isintermingled with a second overlay mark. For example, features 702 fromthe first overlay mark are positioned in rows. Similarly features 704from the second overlay mark are positioned in rows. The rows ofdifferent types of features 702, 704 alternate. When the features 702are misaligned from features 704, the characteristics of acoustic waveseither being transmitted through or reflected off of the features 702,704 will indicate the nature of the misalignment.

FIG. 7C is a diagram showing an example in which a first overlay mark isintermingled with a second overlay mark. For example, features 702 fromthe first overlay mark are clustered in the top left and the bottomright. Features 704 from the second overlay mark are clustered at thetop right and the bottom left. Such a configuration may also allow fordetermining alignment in more than one direction. For example, alignmentmay be determined in one direction, and a second direction orthogonal tothe first direction. When the features 702 are misaligned from features704, the characteristics of acoustic waves either being transmittedthrough or reflected off of the features 702, 704 will indicate thenature of the misalignment.

FIG. 7D is a diagram showing an example in which a first overlay mark isadjacent a second overlay mark in a diagonal manner. For example,features 702 from the first overlay mark are positioned at the bottomleft. Features 704 from the second overlay mark are positioned at thetop right. Such a configuration may also allow for determining alignmentin more than one direction. For example, alignment may be determined inone direction, and a second direction orthogonal to the first direction.When the features 702 are misaligned from features 704, thecharacteristics of acoustic waves either being transmitted through orreflected off of the features 702, 704 will indicate the nature of themisalignment.

FIG. 7E is a diagram showing an example in which a first overlay mark isintermingled with a second overlay mark. For example, features 702 fromthe first overlay mark are positioned in rows. Similarly features 704from the second overlay mark are positioned in rows. The rows ofdifferent types of features 702, 704 alternate. However, there are morerows of features 704 than there are rows of features 702. When thefeatures 702 are misaligned from features 704, the characteristics ofacoustic waves either being transmitted through or reflected off of thefeatures 702, 704 will indicate the nature of the misalignment.

FIG. 7F is a diagram showing an example in which a first overlay mark isintermingled with a second overlay mark. For example, features 702 fromthe first overlay mark are positioned in rows. Similarly features 704from the second overlay mark are positioned in rows. The rows ofdifferent types of features 702, 704 alternate. However, there are morerows of features 704 than there are rows of features 702. FIG. 7F issimilar to FIG. 7E, except the rows run in a different direction. Whenthe features 702 are misaligned from features 704, the characteristicsof acoustic waves either being transmitted through or reflected off ofthe features 702, 704 will indicate the nature of the misalignment.

FIG. 7G is a diagram showing an example in which a first overlay mark isintermingled with a second overlay mark. For example, features 702 fromthe first overlay mark and features 704 from the second overlay mark arepositioned in a checkered pattern. Such a configuration may also allowfor determining alignment in more than one direction. For example,alignment may be determined in one direction, and a second directionorthogonal to the first direction. When the features 702 are misalignedfrom features 704, the characteristics of acoustic waves either beingtransmitted through or reflected off of the features 702, 704 willindicate the nature of the misalignment.

FIG. 8 is a diagram showing various periodic structures, acoustic wavetransmitters and acoustic wave receivers. According to the presentexample, four different overlay marks 802, 804, 806, 808 are positionedin a square pattern within a periodic structure region 110. In someexamples, each of the overlay marks may be associated with a differentmaterial layer. In some examples, two of the overlay marks may beassociated with the same material layer and two of the overlay marks maybe associated with a different material layer. The overlay marks 802,804, 806, 808 may each comprise periodic structures having a pluralityof features in a periodic pattern, such as the patterns described above.

In one example, device 812 is an acoustic wave transmitter deviceconfigured to transmit an acoustic wave across overlay marks 802, 804.Such acoustic wave may be picked up by device 814. Similarly, device 816may be an acoustic wave transmitter device configured to transmit anacoustic wave across overlay marks 806, 808. Such acoustic wave may bepicked up by device 818. Furthermore, device 820 may be an acoustic wavetransmitter device configured to transmit an acoustic wave acrossoverlay marks 802, 806. Such acoustic wave may be picked up by device824. Similarly, device 822 may be an acoustic wave transmitter deviceconfigured to transmit an acoustic wave across overlay marks 804, 808.Such acoustic wave may be picked up by device 826. Thus, theconfiguration shown in FIG. 8 may be used to detect

FIG. 9 is a diagram showing multiple overlay mark regions in series.According to the present example, a plurality of overlay regions 900 a,900 b, 900 c are positioned in series. In other words, they arepositioned along a line that runs in the direction in which acousticwaves are transmitted throughout the overlay regions 900 a, 900 b, 900c. Each overlay region 900 a, 900 b, 900 c includes an acoustic wavetransmitter device, an acoustic wave receiver device, and at least twooverlay marks comprising periodic structures.

FIG. 10 is a diagram showing multiple overlap mark regions in parallel.According to the present example, a plurality of overlay regions 1000 a,1000 b, 1000 c are positioned in parallel. In other words, they arepositioned along a line that runs orthogonal to the direction in whichacoustic waves are transmitted throughout the overlay regions 1000 a,1000 b, 1000 c. Each overlay region 1000 a, 1000 b, 1000 c includes anacoustic wave transmitter device, an acoustic wave receiver device, andat least two overlay marks comprising periodic structures.

FIGS. 11A to 11D are diagrams showing illustrative formation of overlaymarks. According to the present example, FIG. 11A illustrates twodifferent sections of a fabrication wafer 1102. The first section is aproduction section 1101 and the second section is a test section 1103.The production section 1101 is a portion of the wafer on whichintegrated circuits are fabricated. The test section 1103 is a portionof the wafer in which overlay marks may be formed to test the alignmentof patterns of different layers formed in the production section 1101.For purposes of the present discussion, test section 1103 may correspondto the periodic structure region 112 described above in the textaccompanying FIG. 1.

According to the present example, the test section 1103 includes a pieceelectrically 1104, a first hard mask layer 1108, and a second hard masklayer 1106. As described above, overlay marks may be positioned over apiezoelectric layer. A piezoelectric material may experience mechanicalstrain or stress upon application of an electrical current. Thus, an ACelectrical signal applied to the acoustic wave transmitter device maycause surface acoustic wave signal to be transmitted across the testsection 1103.

At the same time, the production section 1101 may have a first materiallayer 1114 to be patterned. The first material layer 1114 may be one ofa variety of materials. For example, the first material layer 1114 maybe of polysilicon layer to form gate structures, or a metal layer toform interconnects. After the first material layer 1114 has beendeposited, a first patterned photoresist layer is deposited, exposed toa light source through a mask, and developed. After development,photoresist features 1110 remain in the production section 1101, andphotoresist features 1112 remain in the test section. Photoresistfeatures 1112 may correspond to a first overlay mark that includesplurality of periodic structures.

FIG. 11B illustrates an etching process 1120 to pattern both the secondhard mask 1106 in the test section 1103 and the first material layer1114 in the production section 1101. The hard mask 1106 may be selectedto have a material that is removable in the same etching process used topattern the first material layer 1114 in the production section. Afterthe etching process, the photoresist features 1110, 1112 may be removed.

FIG. 11C illustrates additional layers. Specifically, FIG. 11Cillustrates an interlayer dielectric layer 1132 deposited over the firstmaterial layer features in the production section 1101. The productionsection 1101 also includes a second material layer 1134 deposited overthe interlayer dielectric layer 1132. The second material 1134 may beone of a variety of materials. For example, the second material layer1134 may be a metal material to form metal lines or vias.

A second photoresist layer is then deposited. The second photoresistlayer, after being exposed and developed, includes photoresist features1130 in the production section and photoresist features 1136 in the restsection. The photoresist features 1136 correspond to a second overlaymark structure that includes a plurality of periodic features. The firstphotoresist features 1130 and second photoresist features 1136 areformed using the same mask. That mask is designed such that when thefirst photoresist features 1130 are aligned with the features of thefirst material layer 1114, the photoresist features 1136 are positionedsuch that they form a unitary overlay mark with the features of thesecond hard mask 1106 (e.g., as shown in FIG. 2A). In other words, allfeatures of both the first hard mask features 1106 and the secondphotoresist features 1136 will have similar spacing. This creates thedesired frequency signals in the detected acoustic waves passing throughthe features. If the first photoresist features 1130 are not properlyaligned with the first material layer (e.g., as shown in FIGS. 2B and2C), then the photoresist features 1136 will not be spaced properly withrespect to the features of the second hard mask 1106.

FIG. 11D illustrates an etching process 1140. After the photoresistfeatures 1130, 1136 are formed, an etching process may be applied topattern the second material layer 1134 and the first hard mask layer1108. The features of the second hard mask layer 1106 and thephotoresist features 1136 define the pattern that will be transferred tothe first hard mask 1108. In some examples, the etching process 1140 maybe selected along with the materials of the first hard mask 1108 and thesecond material layer 1134 so that both the first hard mask 1108 and thesecond material layer 1134 are removed by the same etching process.

FIG. 11D illustrates a state after the etching process 1140 and afterthe photoresist features and second hard mask features have beenremoved. After such features have been removed, the remaining features1142, 1144 of the first hard mask layer 1108 remain. Features 1142 maycorrespond to a first overlay mark (e.g., 108, FIG. 1) and the features1144 may correspond to a second overlay mark (e.g., 110, FIG. 1). Afterthe features 1142, 1144 are formed, acoustic signals may be transmittedacross the features 1142, 1144 to determine whether the features 1134are properly aligned with the features of the first material layer 1114.

FIG. 12 is a flowchart showing an illustrative method for using acousticwaves to detect overlay error. According to the present example, themethod 1200 includes a process 1202 for forming a first material layeron a semiconductor wafer, the first material layer comprising a firstperiodic structure within an overlay mark region of the semiconductorwafer. According to the present example, the method 1200 includes aprocess 1204 for forming a second material layer on the semiconductorwafer, the second material layer comprising a second periodic structurein the overlay mark region. According to the present example, the method1200 includes a process 1206 for, with an acoustic transmitter devicedisposed within the overlay mark region, transmitting an acoustic waveacross both the first periodic structure and the second periodicstructure. According to the present example, the method 1200 includes aprocess 1208 for, with an acoustic wave receiver device, detecting theacoustic wave. According to the present example, the method 1200includes a process 1210 for determining an overlay error between thefirst material layer and the second material layer based on the acousticwave as detected by the acoustic wave receiver device.

According to one example, a structure includes a first periodicstructure positioned on a chip, the first periodic structure comprisinga material of a first layer disposed on the chip. The structure furtherincludes a second periodic structure positioned within the region of thechip adjacent the first periodic structure, the second periodicstructure comprising a second material of a second layer disposed on thechip. The structure further includes an acoustic wave transmitter devicedisposed on the chip and an acoustic wave receiver device disposed onthe chip.

According to one example, a device includes an acoustic wave transmitterpositioned on a chip, an acoustic wave receiver positioned on the chip,and a first periodic structure positioned on the chip, the firstperiodic structure comprising a first material. The device furtherincludes a second periodic structure positioned on the chip, the secondperiodic structure comprising a second material.

According to one example, a method includes forming a first materiallayer on a semiconductor wafer, the first material layer comprising afirst periodic structure within an overlay mark region of thesemiconductor wafer. The method further includes forming a secondmaterial layer on the semiconductor wafer, the second material layercomprising a second periodic structure in the overlay mark region. Themethod further includes, with an acoustic transmitter device disposedwithin the overlay mark region, transmitting an acoustic wave acrossboth the first periodic structure and the second periodic structure. Themethod further includes, with an acoustic wave receiver device,detecting the acoustic wave. The method further includes determining anoverlay error between the first material layer and the second materiallayer based on the acoustic wave as detected by the acoustic wavereceiver device.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: forming a first materiallayer on a semiconductor wafer, the first material layer comprising afirst periodic structure within an overlay mark region of thesemiconductor wafer; forming a second material layer on thesemiconductor wafer, the second material layer comprising a secondperiodic structure in the overlay mark region; with an acoustictransmitter device disposed within the overlay mark region, transmittingan acoustic wave across both the first periodic structure and the secondperiodic structure; with an acoustic wave receiver device, detecting theacoustic wave; and determining an overlay error between the firstmaterial layer and the second material layer based on the acoustic waveas detected by the acoustic wave receiver device.
 2. The method of claim1, wherein the acoustic wave detected by the acoustic wave receiverdevice is one of: a transmission through the periodic structures or areflectance off of the periodic structures.
 3. The method of claim 1,wherein determining the overlay error comprises analyzing the detectedacoustic wave using one of: scattering frequency count; scatteringangle; full width half maximum, or center frequency.
 4. The method ofclaim 1, wherein determining the overlay error comprises determining anerror in a first direction and a second direction orthogonal to thefirst direction.
 5. The method of claim 1, further comprising,determining a focus curve by analyzing the detected acoustic wave.
 6. Amethod for forming a structure, the method comprising: forming a firstperiodic structure on a chip, the first periodic structure comprising amaterial of a first layer disposed on the chip, wherein the firstperiodic structure comprises a two-dimensional array of features;forming a second periodic structure within a region of the chip adjacentthe first periodic structure, the second periodic structure comprising asecond material of a second layer disposed on the chip; forming anacoustic wave transmitter device on the chip; and forming an acousticwave receiver device on the chip.
 7. The structure of claim 6, whereinthe first period structure and the second periodic structure arepositioned between the acoustic wave transmitter device and the acousticwave receiver device.
 8. The method of claim 6, further comprising: withthe acoustic wave transmitter device, transmitting an acoustic waveacross both the first periodic structure and the second periodicstructure; with the acoustic wave receiver device, detecting theacoustic wave; and determining an overlay error between the firstmaterial layer and the second material layer based on the acoustic waveas detected by the acoustic wave receiver device.
 9. The method of claim6, wherein features within the array of features have one of: arectangular shape, an elliptical shape, a circular shape, or a squareshape.
 10. The method of claim 6, wherein features of the first periodicstructure are intermingled with features of the second periodicstructure.
 11. The method of claim 10, wherein the features of the firstperiodic structure and the features of the second periodic structure arepositioned in alternating rows.
 12. The method of claim 10, wherein thefeatures of the first periodic structure and the features of the secondperiodic structure are positioned in a checkered pattern.
 13. The methodof claim 6, wherein features of the first periodic structurecircumscribe features of the second periodic structure.
 14. The methodof claim 6, further comprising, a piezoelectric layer underneath thefirst periodic structure and the second periodic structure.
 15. Themethod of claim 6, further comprising, regions of the chip, each regioncomprising additional periodic structures, an additional acoustic wavereceiver, and an additional acoustic wave transmitter, the regions beingpositioned in series.
 16. The method of claim 6, further comprising,regions of the chip, each region comprising additional periodicstructures, an additional acoustic wave receiver, and an additionalacoustic wave transmitter, the regions being positioned in parallel. 17.A method comprising: forming an acoustic wave transmitter on a chip;forming an acoustic wave receiver on the chip; forming a first periodicstructure on the chip, the first periodic structure comprising a firstmaterial; and forming a second periodic structure on the chip, thesecond periodic structure comprising a second material; wherein theacoustic wave transmitter device is configured to transmit an acousticwave across both the first periodic structure and the second periodicstructure.
 18. The method of claim 17, wherein the acoustic wavetransmitter comprises an interdigital transducer.
 19. The method ofclaim 17, wherein the acoustic wave transmitter comprises an acousticemitter.
 20. The method of claim 17, further comprising: with theacoustic wave transmitter, transmitting an acoustic wave across both thefirst periodic structure and the second periodic structure; with theacoustic wave receiver, detecting the acoustic wave; and determining anoverlay error between the first material layer and the second materiallayer based on the acoustic wave as detected by the acoustic wavereceiver.